This invention relates generally to a thermal transfer ink sheet and a method of printing and, more specifically to ink sheets including a fusible ink and a heat resistant film loaded with the ink, suitable for transferring images onto recording media with rough surfaces.
Conventional methods of thermal transfer printing utilize a ribbon shaped or web shaped ink sheet or support made from a heat resistant film coated with fusible ink. A thermal transfer recording apparatus prints by heating specific portions of the ink sheet to melt the fusible ink and transfer the ink at specific portions to the recording medium. The printing sheet is stripped off the recording medium leaving the previously softened portions of ink on the recording medium in the form of dots of ink called pixels.
FIGS. 8A and 8B illustrate conventional thermal printing. Using an ink transfer film 80 including a heat resistant film c having a fusible ink layer b disposed thereon, ink transfer film 80 is interposed between a print head p and a recording medium or paper s. Print head p is heated electrically where printing is desired. Heat passes through heat resistant film c into ink layer b to soften the ink directly between print head p and paper s. Ink sheet 80 is then removed in the direction of arrow e and printed portions b' remain affixed to the surface of paper s.
If paper s is rough, it will have a plurality of convex hills d which contact ink layer b during printing and a plurality of concave valleys g therebetween which do not contact ink layer b. Heat from print head p is conducted from ink layer b at hills d of paper s where ink layer b contacts paper s. Heat is not conducted to concave valleys of paper s. Accordingly, ink will not flow into valleys q and flows towards the heated convex portions d on the surface of paper s which are higher in temperature.
When ink sheet c is removed from recording paper s, as shown in FIG. 8B, recording paper s has non-uniform clumps of ink on convex hills d and voids over concave valleys g. The voids lead to lower optical density of the printed image. The non-uniform printed surface causes irregular refraction of light. To correct this problem, prior art methods have substituted inks with low fluidity to prevent ink from flowing onto the raised portions of rough surfaced paper. Such inks require more thermal energy to accomplish the transfer process. Further, these less fluid inks do not adhere to the recording paper as well as the more fluid inks. Thus, such attempts have not been fully satisfactory.
Accordingly, it is desirable to provide improved thermal transfer ink sheets which overcome the deficiencies of the prior art by providing an ink sheet capable of forming uniform images with high dot concentration on rough surfaces recording media while using a low level of thermal printing energy. Similarly, it is desirable to print high density uniform images with an improved thermal transfer method.